Electrically programmable read-only memories (EPROMs) are well understood in the prior at. These memories typically comprise an array of individual EPROM cells which store information in the form of an electrical charge. An example of a cell for such memories is described in U.S. Pat. No. 3,996,657. In many cases, these EPROM memories can be erased by exposure to ultraviolet radiation. Memory arrays which employ cells that are erasable upon exposure to ultraviolet radiation are disclosed in U.S. Pat. Nos. 3,728,695 and 3,744,036.
It is widely recognized that employing redundant rows and columns is beneficial for replacing defective rows and columns of cells in prior art EPROM arrays. Most often, fusible silicon links are used to enable these redundant elements. When used in memory arrays which can be erased by ultraviolet radiation, the integrated circuit must include some means for protecting these redundant areas from radiation. In the past, this has been accomplished by shielding the redundant EPROM cells with a metal covering layer. An example of a radiation shield for an integrated circuit memory with redundant elements is disclosed in U.S. Pat. No. 4,519,050 (hereinafter the '050 patent), which is also assigned to the assignee of the present invention.
The radiation shield described in the '050 patent suffers from several drawbacks. To begin with, the '050 patent teaches the use of buried contacts in a single metal-oxide-semiconductor (MOS) fabrication process. These contacts are utilized for providing electrical contact between the n+drain diffusion regions and a polysilicon conductor. The polysilicon conductor spiral out of the EPROM cell underneath the overlying metal protective shield, and provide connection to extend circuits in the memory.
The fabrication sequence required to produce these buried contacts is complex and requires numerous processing steps. A further drawback of buried contacts is that they exhibit a high resistivity and are generally unreliable over the lifetime of the part. However, eliminating buried contacts still leaves the problem of making electrical connection between the drain region and the other devices within the EPROM memory. In a single metal layer process, it has proven difficult to provide a metallic radiation shield while at the same time providing metalized contacts to the underlying diffusion regions in the substrate.
One alternative to the use of buried contacts is to form an n-well region under the radiation shield to provide direct connection to the n+drain diffusion region via the substrate. However, this approach suffers from the fact that the n-well conductor is highly resistive, rendering this approach impractical. Therefore, what is needed is an improved radiation shield for electrically programmable read-only storage devices which obviates the need for buried contacs and which provides additional flexibility in the patterning of the interconnect lines.